This all new bellmouth for the popular Dellorto SI 20mm carb series is definitely an innovative product.
The SI carb series is so popular for Vespa engines because of its low dimension, spare part availability and short intake dimension.
To further enhance power Polini has developed this inlet in the venturi-design. The venturi accelerates the mixture. The form of the new inlet is not plain like the SI one, but styled like conventional carbs with a narrowing.
The narrowing of the venturi in conjunction with the larger surface area, that is not restricted by an air filter, that sits directly at the bellmouth, enhances the filling grade with mixture and boosts power this way.
The foam filter is fitted instead of the elbow to protect the engine against dirt and wear.
The sound is a little crispier. It is advisable to check the carb box gaskets for tightness.
It is obvious that more mixture and power means less fuel efficiency and that the carb needs to be set up new after the modification. Several main jet sets and other tuning parts are available for this.
With this conversion the si carb delivers impressive power only known from other and bigger, more conventional carb set ups.
The better, inner cooling of the surplus mixture helps to cool down thermal stressed engines as well.
A fresh float valve, new gaskets and in some cases a spacer for the float chamber are a good idea for efficient fuel flow.
On engines with very high power output the main jet might excess the 160+ region. Jets of that size are not build by Dellorto and have to be handmade using a jet reamer or similar.Please note: Always secure the fixing screws of the bell mouth with liquid threadlock.